A fiber optic gyroscope (FOG) is a device used to sense rotation by detecting phase shift differences between two interfering, counter-propagating beams of light. As is well-known to those of ordinary skill in the art, FOGs exhibit bias offset errors in the presence of magnetic fields. These bias offset errors are caused by fiber twist, which occurs naturally and inevitably during the FOG manufacturing process.
One common approach for reducing the influence of magnetic fields is to encase certain components of the FOG in a housing comprising magnetic shielding material, which is designed to protect sensitive components of the FOG from magnetic field effects. This approach, however, presents a number of drawbacks. For example, such magnetic shielding material increases both the weight and the cost of a FOG. In addition, it is typically infeasible to encase a FOG with sufficient shielding material to completely isolate sensitive components from the influence of ambient magnetic fields.
Another approach for reducing the influence of magnetic fields is to add optical components, or compensators, to the FOG which are specifically designed to compensate for the bias offset errors experienced in the presence of ambient magnetic fields. This approach typically involves a complex and demanding fabrication process in which, first, the axial and tangential components of the overall magnetic sensitivity of a FOG are determined. Then, two discrete compensators are designed and fabricated to exactly match and cancel each of these components independently, i.e., a first compensator is designed and fabricated to cancel the axial magnetic sensitivity component, and a second compensator is designed and fabricated to cancel the tangential magnetic sensitivity component.
This approach also presents a number of drawbacks. For example, the approach requires that each compensator be fabricated in accordance with extremely tight tolerances both for the magnitude and the orientation of its sensitivity. In addition, by using this approach, the assembly process for a FOG involves tight tolerances on the mechanical alignment of the compensators, both to assure cancellation of the sensitivity along the axis of the compensator and to avoid introducing sensitivity along the orthogonal axes. Given the tight tolerances associated with this approach, the result is a very labor-intensive and iterative process to continually adjust the sensitivities until an acceptable compensation level is achieved. Moreover, even using state-of-the-art fabrication techniques, there are limitations on the ability to fabricate compensators with specific sensitivities. As a result, the ultimate performance of such compensators is limited.